Design Of High Strength Steel Research Articles

Behaviour, finite element modelling and design of high strength steel homogeneous and hybrid welded I‐section beams

AbstractThe behaviour and design of homogeneous and hybrid high strength steel (HSS) beams are addressed in the present study. Six in‐plane three‐point bending tests on three different welded I‐sections were first conducted. Following the experimental investigation, a finite element (FE) modelling programme was performed, which included a validation study confirming the accuracy of the developed FE models in replicating the flexural behaviour of HSS welded I‐section beams, and a parametric study generating additional FE data on HSS welded I‐section beams over a broader range of cross‐sectional slendernesses, steel grades and loading configurations. Then, the suitability of the current Eurocode 3 cross‐section slenderness limits for HSS homogeneous and hybrid welded I‐sections in bending were evaluated. It is shown that the current Eurocode Class 2 and Class 3 slenderness limits are suitable for the classification of the outstand flange (in compression) and internal web (in bending) elements of both HSS homogeneous and hybrid welded I‐sections subjected to major axis bending, while stricter Class 1 slenderness limits are considered necessary to achieve sufficient rotation capacity for plastic design. The findings indicate that plastic design can be used for HSS structures, provided the proposed stricter Class 1 slenderness limits are employed. Further work is underway to develop advanced design approaches using second‐order inelastic analysis with strain limits for HSS welded I‐section members.

Response of hydrogen diffusion and hydrogen embrittlement to Cu addition in low carbon low alloy steel

Hydrogen embrittlement (HE) has arisen as a critical issue for the development of high strength steel. The austenite and Cu-rich precipitates can provide stable trapping sites for diffusible hydrogen, and thus improved the resistance to HE. But the research on the synergistic effect of Cu and austenite on HE in low carbon low alloy steel has not been actively reported. In this study, four steels with varying Cu addition (0-3 wt%) were fabricated, and their resistance to HE was evaluated. The role of Cu addition was investigated by the ductile-brittle transition temperature (DBTT) value obtained from Charpy impact test and the loss of elongation measured from slow-strain-rate tensile (SSRT) test after hydrogen charging. Also, the hydrogen diffusivity and concentration were evaluated by electrochemical hydrogen permeation test. Cu addition resulted in a first decrease and subsequent increase of effective diffusion coefficient (D eff ), elongation loss and DBTT, and the minimum value was achieved in steel with 1.5 wt% Cu addition, indicating a higher resistance to HE. The unraveled mechanism was that Cu addition increased the content of retained austenite, which impeded the diffusion of hydrogen during deformation. But the stability of retained austenite decreased when the content of retained austenite reached a high value, which led to martensitic transformation and accordingly increased HE susceptibility. Moreover, Cu addition promoted the formation of Cu-rich precipitates in which Cu cluster/bcc Cu-rich precipitate coherent/semi-coherent with matrix can be strong trapping sites for hydrogen, and fcc Cu-rich precipitate acted as weak trapping sites for hydrogen. In addition, a higher density of dislocations was found in the steel with 1.5 wt% Cu addition, which may contribute to lower D eff and higher density of hydrogen traps (N T ). The present work suggested that Cu addition can enhance the resistance to HE during tensile and impact processes for the design of high strength steel. • Cu addition resulted in a first decrease and subsequent increase in the effective diffusion coefficient (D eff ) of hydrogen. • Cu addition increased the elongation rate and decreased the ductile-brittle transition temperature (DBTT) of the steels after hydrogen embrittlement. • The resistance to hydrogen embrittlement during tensile and impact process was determined by retained austenite, Cu-rich precipitates and dislocations in the microstructure. • The resistance to hydrogen embrittlement varied with the content and stability of austenite and the structure of Cu-rich precipitates.

Behaviour and design of high strength steel homogeneous and hybrid welded I-section beams

The behaviour and design of high strength steel (HSS) beams are addressed in the present study. Six in-plane three-point bending tests on three different welded I-sections − two homogeneous S690 steel welded I-sections and one hybrid welded I-section with S690 steel flanges and an S355 steel web, were first conducted. The beam tests were carried out in major axis bending and a bespoke restraint system was designed and employed in the test programme to prevent lateral-torsional buckling. Following the experimental investigation, a thorough finite element (FE) modelling programme was performed, which included a validation study confirming the accuracy of the developed FE models in replicating the flexural behaviour of HSS welded I-section beams, and a parametric study generating additional FE data on HSS welded I-section beams over a broader range of cross-sectional slendernesses, steel grades and loading configurations. The test results obtained in the present study and collected from the literature, together with the generated FE data from the parametric study, were used to evaluate the suitability of the current Eurocode 3 cross-section slenderness limits for HSS homogeneous and hybrid welded I-sections in bending. It is shown that the current Eurocode Class 2 and Class 3 slenderness limits are suitable for the classification of the outstand flange (in compression) and internal web (in bending) elements of both HSS homogeneous and hybrid welded I-sections subjected to major axis bending, while stricter Class 1 slenderness limits are considered necessary to achieve sufficient rotation capacity for plastic design. The findings from the present study indicate that plastic design can be used for HSS structures, provided the proposed stricter Class 1 slenderness limits are employed.

Machine-learning-based design of high strength steel bolted connections

For the design of high strength steel bolted connections, all existing standards adopt the same framework – (i) calculating the design resistance for each potential failure mode and (ii) defining the final design failure load as the minimum of the design resistances calculated from all the potential failure modes. However, this framework has been found to be tedious and also incapable of considering the complex nature of connection behaviour, in particular the transition of different failure modes (e.g., net section fracture, bearing and block tearing), and thus leads to inaccurate failure load and mode predictions. To address the aforementioned shortcomings, this paper presents a more accurate and reliable predictive framework based on machine learning. Firstly, a database including 543 experimental and numerical data was collected. Then, regression models for failure load predictions and classification models for failure mode predictions were developed based on eight machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Light Gradient Boosting Machine, Extreme Gradient Boosting and Cat Boosting. The performance of the developed models was assessed based on the collected data, indicating that the Support Vector Machine models led to the best predictions of failure loads and modes. The Support Vector Machine models were then compared with existing design standards and shown to yield substantially improved failure load and mode predictions for high strength steel bolted connections. Specifically, the mean ratio of experimental and numerical to predicted failure loads from the machine-learning-based approach is equal to 1.00, while the corresponding mean ratios from the design standards range from 1.10 to 1.39. The machine-learning-based approach is capable of accurately predicting 97.2% of the total failure modes, while the design standards can only accurately predict 67.9%–85.3% of the total failure modes.

Interactions of nanoprecipitates, metastable austenite, and Lüders banding in an ultra-low carbon medium Mn steel with abnormal strength dependence of hydrogen embrittlement

A hierarchical microstructure, containing an ultrafine-grained matrix with mixed lamellar and equiaxed morphology, metastable austenite, and intermetallic nanoprecipitate, was designed to improve the hydrogen embrittlement (HE) resistance of ultra-low carbon medium Mn steels. Compared with the warm-rolled (WR) state, the warm-rolled-tempering (WRT) sample shows a simultaneous enhancement of yield strength (YS) and tensile ductility, but lower total elongation loss (∼3%) after hydrogen charging, thus exhibiting an abnormal strength dependence of HE. The microstructural analysis with numerical calculation proves that an additional tempering not only introduces a heterogeneous distribution of Ni(Al, Mn) precipitations, but also results in a reverse γ→α transformation with Mn partitioning, thus contributing to a self-stabilization of austenite with refined grain size and enriched element stabilizer. On one hand, the introduction of intermetallic Ni(Al, Mn) precipitations could be beneficial for the intrinsic hydrogen capture and storage, although its limited number density only contributes little to impeding hydrogen diffusion. Moreover, the introduction of nanoprecipitate can improve the strain compatibility and suppress the strain concentration in ferrite phases, which is consistent with a lower kernel average misorientation (KAM) value of the WRT sample after deformation. Meanwhile, the increased austenite stability leads to a shorter yield point elongation (YPE) and an effective transformation-induced plasticity (TRIP) effect at large strains, and the transformation product of nano-lamellar α’-martensite from self-stabilized austenite contributes little to the hydrogen-accelerated strain localization in weak α/γ interfaces of the WRT sample. In contrast, the low HE resistance of the WR sample is mainly attributed to the enhanced martensitic transformation process during the propagation of Lüders banding, which leads to a strong localized stress concentration in α/α’ interfaces, and the premature failure due to hydrogen-enhanced decohesion effect (HEDE). The interactions of nanoprecipitate, metastable austenite, and Lüders banding provide new insight into the design of high-strength steel with excellent HE resistance.

Composition and processing of direct-quench hot rolled steels with ultrahigh strength exceeding GPa

The design of high-strength steel has long been discussed in the field of metal structural materials. To further increase the strength of common high-strength steel and further decrease the cost for production, three direct-quench hot rolled steels were designed and fabricated. The rolling and coiling processes were set based on continuous cooling transformation curves. In addition, the effect of the coiling temperature on the tensile properties was discussed to further guide the optimization of the process. It was found that compared with granular bainite, lower bainite probably has more advantages for both the strength and low temperature impact toughness of direct-quench hot rolled steels. Through a process of tailoring the morphology of bainite and controlling the grain boundary precipitation, the newly designed direct-quench hot rolled steels showed greatly improved strength and acceptable ductility/toughness compared with traditional quenched and tempered steels.

Behaviour and design of high-strength steel beam-to-column joints

This paper presents a finite element model for predicting the behaviour of high-strength steel bolted beam-tocolumn joints under monotonic loading. The developed numerical model considers the effects of material nonlinearities and geometric nonlinearities. The accuracy of the developed model is examined by comparing the predicted results with independent experimental results. It is demonstrated that the proposed model accurately predicts the ultimate flexural resistances and moment-rotation curves for high-strength steel bolted beam-to-column joints. Mechanical performance of three joint configurations with various design details is examined. A parametric study is carried out to investigate the effects of key design parameters on the behaviour of bolted beam-to-column joints with double-extended endplates. The plastic flexural capacities of the beam-to-column joints from the experimental programme and numerical analysis are compared with the current codes of practice. It is found that the initial stiffness and plastic flexural resistance of the high-strength steel beam-to-column joints are overestimated. Proper modifications need to be conducted to ensure the current analytical method can be safely used for the bolted beam-to-column joints with high-performance materials.

Mixed mode fatigue crack growth behavior of Ni-Cr-Mo-V high strength steel weldments

The extensive use of Ni-Cr-Mo-V high strength steels in modern marine environment poses increasing requirements for accurate characterization of the fatigue crack growth (FCG) behavior of their welded joints. The present work reported a study on a new generation Ni-Cr-Mo-V high strength steel concerning the mixed mode FCG behavior of its welded joint. Effects of microstructure and welding residual stresses (WRSs) were emphasized. It was found that the 4% compliance offset criterion in the conventional ASTM procedure and the 10% offset criterion in the normalized ASTM procedure were suitable for determination of the crack opening force of high strength steel weldments. A new effective stress intensity factor range parameter as the crack growth driving force was proposed, which yielded fairly good correlation for the mixed mode FCG data. Acicular ferrite with basket-weave microstructure in the weld metal exhibited favorable crack growth resistance relative to the base material and was preferred to the welded joints. The attained master or intrinsic FCG curves for the base material and the weld metal on the basis of the proposed parameter provided a basic database for the design of high strength steels employed in modern marine structures.

Cross-sectional analysis of arbitrary sections allowing for residual stresses

The method of cross-section analysis for different sections in a structural frame has been widely investigated since the 1960s for determination of sectional capacities of beam-columns. Many handcalculated equations and design graphs were proposed for the specific shape and type of sections in precomputer age decades ago. In design of many practical sections, these equations may be uneconomical and inapplicable for sections with irregular shapes, leading to the high construction cost or inadequate safety. This paper not only proposes a versatile numerical procedure for sectional analysis of beam-columns, but also suggests a method to account for residual stress and geometric imperfections separately and the approach is applied to design of high strength steels requiring axial force-moment interaction for advanced analysis or direct analysis. A cross-section analysis technique that provides interaction curves of arbitrary welded sections with consideration of the effects of residual stress by meshing the entire section into small triangular fibers is formulated. In this study, two doubly symmetric sections (box-section and H-section) fabricated by high-strength steel is utilized to validate the accuracy and efficiency of the proposed method against a hand-calculation procedure. The effects of residual stress are mostly not considered explicitly in previous works and they are considered in an explicit manner in this paper which further discusses the basis of the yield surface theory for design of structures made of high strength steels.

Hot rolling bonded multilayered composite steels and varied tensile deformation behaviour

Multilayered metal composites consisting of alternating metals or alloys have been well developed to obtain the superior mechanical properties different from those in any of the constituent materials. The layer integrated steel and metals research project sponsored by Japan government started from 2006 and aimed to fabricate steel sheets with high strength (>1200 MPa) and superior elongation (>20%), through the composite design of high strength steels and ductile steel layers. In the present work, two kinds of hot rolling bonded multilayered composites consisting of austenitic stainless steel/bainite (martensite) steel are focused. The effects of hot rolled microstructure and the annealed microstructure on the tensile behaviour of a 25-layer laminated composite steel are discussed. On the other hand, the tensile deformation microstructure of the hot rolling bonded and annealed 15-layer composite steel is systematically investigated in order to find the reason for the enhanced plasticity of the brittle layer in the composite.

Design of high-strength steels by microalloying and thermomechanical treatment

Steels with higher strength, ductility and improved fatigue behavior are required for light-weight structures in the transportation industry. It is shown that for martensitic steels the combination of microalloying and an optimized thermomechanical treatment (TMT) results in increased strength and improved ductility. Proper conditioning of the austenite by deformation either refines the austenitic grains or generates a dislocation substructure that is inherited to the martensite structure. In contrast to simply quenched and tempered martensite with no prior deformation, the thermomechanically processed martensite exhibits a more refined structure with refined blocks and is free of grain boundary carbides. Addition of vanadium is beneficial in controlling the austenite grain size during austenitization and for the stabilization of the austenite defect structures that are produced by deformation. It enables to use higher deformation temperatures for TMT, i.e. lower rolling forces can be applied in an industrial process. It is possible to increase the strength and ductility of conventionally heat treated Si–Cr steel by addition of vanadium or by TMT, but the highest improvement is achieved through the combination of both. In this study, an increase of more than 600 MPa in the ultimate tensile strength and an improvement of 40% in the reduction area are reported.

Modelling Upper and Lower Bainite Trasformation in Steels

Bainite is of considerable importance in the design of high strength steels. There are two types of morphologies, upper and lower bainite. In upper bainite, cementite forms between adjacent bainitic ferrite plates. In certain steels, however, the cementite reaction is suppressed so that carbon-enriched austenite remains untransformed between bainitic ferrite plates. In lower bainite, cementite also has the opportunity to precipitate within bainitic ferrite plates. In order to model the development of these microstructures, it is necessary to treat the simultaneous formation of both the ferritic and carbide components of the microstructure. A theory has been developed to do exactly this, enabling the estimation of the phase fractions, the cementite particle size and the transition from upper to lower bainite. The results have been compared against experimental data.